A single lens which is made of glass or plastic material having a homogeneous refractive index and which has spherical or flat surfaces is incapable of correcting the spherical aberration on the optical axis. Although lenses having an aspherical surface is capable of correcting the spherical aberration, such aspherical lenses have problems that molds used for press-molding those lenses have a high production cost and that grinding marks remained on the mold surface are transferred to the lens surface.
Another way to correct spherical aberration is to use an axial refractive index distributed lens. FIGS. 1(a) and (b) are a sectional view diagrammatically showing a planoconvex condenser lens 1 with an axial gradient refractive index distribution and a graph showing the gradient refractive index distribution thereof, respectively.
The axial refractive index distributed lens 1 gives a refractive index distribution R in the direction of the optical axis L thereof and has a uniform refractive index within the plane perpendicular to the optical axis. The refractive index distribution in the spherical surface part is linear and has a specific slope according to the radius of curvature, whereby the spherical aberration can be satisfactorily corrected. Lenses having a larger numerical aperture (NA), i.e., lenses which are brighter, should have a larger difference in refractive index within the linear part. Whether or not the spherical surface part outside the area of the effective diameter D has a refractive index distribution has almost no influence on spherical aberration. As compared with aspherical lenses, this axial refractive index distributed lens has an advantage that grinding and polishing are easy, because the lens surface may be spherical or may remain flat.
A preferred method for producing the axial refractive index distributed lens is a method as described below.
As shown in FIG. 2, a flat glass plate 5 having a uniform refractive index is provided, and this glass plate is immersed in a molten salt 6, e.g., a nitrate or sulfate. Ion exchange occurs between the glass plate 5 and the molten salt 6 by diffusion, into the glass plate 5, ions which contribute to the change of refractive index toward the deep inner part from the glass surface. A refractive index distribution which gradually decreases or increases toward the deep inner part of the glass from the surface thereof is formed in the glass plate 5. A part of the glass plate which contains the refractive index distribution is cut out and ground and polished to make an axial refractive index distributed lens.
In the above ion exchange process for producing the axial refractive index distributed lens, it is most suitable to use thallium ions as ions which contribute to a refractive index distribution in order to obtain a large refractive index difference. The molten salt used in this embodiment is a molten salt comprising a thallium salt such as TlNO.sub.3 or Tl.sub.2 SO.sub.4.
However, since thallium is highly toxic, glasses containing a large amount of thallium and the use of a molten salt containing a large amount of thallium pose a problem from the standpoint of environmental pollution.
Another possible ions which contribute to a large refractive index difference are silver ions as shown in JP-A-61-261238 and JP-A-62-100451. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) Since silver ions generally tend to form a colloid, a glass composition containing phosphorus oxide in a large amount so as to inhibit the colloid formation of silver ions is proposed in the above JP-A references.
However, glasses containing a large amount of phosphorus oxide have poor weather resistance and hence pose a problem in practical use. Glasses containing a large amount of phosphorus oxide have another problem that the glasses react with a nitric acid salt during ion exchange process to form a devitrification product on the glass surface or the glasses dissolve in the molten salt. Using a molten salt other than nitric acid salts, e.g., a molten salt of a sulfuric acid salt or a halide, has many problems, for example, that such a molten salt highly corrodes metals and glasses and it is hence difficult to select an appropriate container for the molten salt.
Improvements of the glass composition shown in JP-A-62-100451 include the glass composition described in JP-A-4-2629. This glass composition also is insufficient in stability in molten salts and durability of the glass material, and hence has a problem in practical use.
On the other hand, an aluminosilicate glass is known as a glass composition which does not contain phosphorus oxide and in which silver ions do not form a colloid. In general, when alkali ions are incorporated into a silicate glass, the silicate framework is cut and non-bridging oxygens (hereinafter referred to as "NBO") strongly bonded to alkali ions are formed. If silver ions are incorporated by ion exchange into a glass in which NBO is present, the silver ions incorporated are reduced by the action of NBO to form a silver colloid, which colors the glass. The resulting colored glass cannot be used as a lens. In contrast, when Al.sub.2 O.sub.3 is added to a silicate glass, the Al.sub.2 O.sub.3 is incorporated in the form of AlO.sub.4.sup.-, which combines with an alkali to reduce the amount of NBO in the glass. As a result, silver ions tend to be present stably in the ion form. Since AlO.sub.4.sup.- ions combine with alkali ions in a ratio of 1:1, the amount of NBO in the glass is the smallest (or is zero in some glasses) when [M]/[Al]=1 ([M] and [Al] represent a molar concentration of alkali ions and AlO.sub.4.sup.- ions, respectively, in the glass). Consequently, an aluminosilicate glass in which [M]/[Al]=1 can most stably contain silver ions therein.
Incidentally, in order to obtain the axial refractive index distributed lens with larger NA, it is necessary to make the axial refractive index difference large. To attain this it is necessary to use a glass containing a large amount of alkali ions to be replaced with silver ions, because the refractive index difference is approximately proportional to the silver ion concentration. In order to enable silver ions to be present stably in an aluminosilicate glass for the above purpose, it is necessary to increase the concentration of Al.sub.2 O.sub.3 as the alkali concentration in the glass increases. However, the melting temperature of a glass elevates if the Al.sub.2 O.sub.3 concentration in the glass increases. As a result, it is difficult to produce a glass having preferable quality (free from striae, bubbles, etc.). In order to lower the melting temperature, it is considered to decrease the Al.sub.2 O.sub.3 concentration in the glass. However, the amount of silver ions which can be contained in this glass without forming a colloid is decreased, so that a large refractive index difference cannot be obtained.
It is also known that the melting point of a glass can be lowered without formation of colloid of silver ions by incorporating B.sub.2 O.sub.3 into the glass (Glastech. Ber., 64 [8] 199 (1991), Appl. Opt, 31 [25] 5221 (1992), and J. Non-Cryst. Solids, 113 37 (1989); in glasses, boron has a valence of 3 like aluminum). However, too high B.sub.2 O.sub.3 concentration in a glass poses problems that durability of the glass deteriorates and a rate of silver ion exchange is decreased. Therefore, the concentration of B.sub.2 O.sub.3 which can be incorporated is limited.
Thus, an axial refractive index distributed lens having a large axial refractive index difference and high quality, thus having sufficiently satisfactory properties, has not been provided so far.